OS wont go full screen

I have gotten virtual box up and running. I have been a able to run Ubuntu and Linux mint, but I’m having a problem with going full screen. When the OS is launched the window is about 1/3 the size of my actual screen. I click Machine/full screen and the window gets larger but the actual area where the window was is still there, thus the window where the os is running never changed just took up more space with white area around it. I have tried going to system tools/monitor to change the resolution but im unable to since the monitor is considered unknown by the application. I have also tried scale change but that will only distort the look of it as it reaches full screen.
How can I make the virtual machine go full screen?